A lubrication system for an engine is provided which includes an oil tank, a first passage communicating a bottom part of the crankcase chamber with an upper part of the oil tank, a first check valve provided in the first passage to permit a flow from the crankcase chamber to the oil tank but not in a reverse direction, a second passage communicating an upper part of the oil tank with an upper part of the crankcase chamber, a second check valve provided in the second passage to permit a flow from the oil tank to the crankcase chamber but not in a reserve direction and a third passage communicating a lower part of the oil tank with the second passage.